The Archaeological Institute has some background on what is known about the site so far.
The few artifacts unearthed at the site seem to date it to the mid-eighth century A.D. During this period, Por-Bajin was on the periphery of the Uighur Empire, which lasted from A.D. 742 to 848 and was held together by forces of warriors on horseback.
Were some of those warriors once garrisoned at Por-Bajin? The Uighurs also might have built the site on an island for reasons other than defense. Perhaps the island was the site of a palace or a memorial for a ruler. Por-Bajin's unique layout, more intricate than that of other Uighur fortresses of the period, has led some scholars to suggest that it might have had a ritual role.
Determining how the site was used might also help archaeologists understand why it was abandoned. There is some evidence of a great fire at Por-Bajin, but could there be other reasons the Uighurs eventually left?